Verification of fingertip blood lactate measurement as an index of muscle fatigue recovery

Abstract

[Purpose] To evaluate the validity of measurement sites for assessing blood lactic acid levels following unilateral upper-limb exercises. [Participants and Methods] Blood lactic acid levels were measured at the fingertips of both hands in 40 healthy young men (mean age 20.9 ± 2.5 years). Measurements were taken before, immediately after, and at 90-s intervals following unilateral upper-limb exercises involving palmar flexion and dorsiflexion of the non-dominant hand. Exercise load was determined through maximum voluntary contraction testing. [Results] No significant differences were found in the average blood lactic acid levels between the fingers at any measurement time point. The peak blood lactic acid level occurred approximately 90 s earlier in the motor limb than in the non-motor limb. [Conclusion] This study found no significant difference in blood lactic acid levels between the motor and non-motor limbs when using fingertip measurements during unilateral upper-limb exercises. Therefore, either fingertip may be a suitable measurement site. However, because the peak lactic acid level in the non-motor limb was delayed by around 90 s, the timing of peak muscle fatigue evaluation in the non-motor limb should be considered.

INTRODUCTION

Many patients are affected by unilateral upper limb sports injuries, such as pitcher’s shoulder or tennis elbow^{1, 2)}. Physiotherapists recommend performing strength training to treat these disorders; however, excessive strength training can cause muscle damage^3,4,5). Therefore, it is important to understand muscle fatigue and training programs. If muscle fatigue occurs during training, treatments, such as electrical stimulation, can promote recovery⁶⁾. Hence, if muscle fatigue can be immediately assessed at the time of training, management can be performed quickly, enabling rapid recovery from fatigue and an eventual return to the state at an early stage.

Quantitative assessment of muscle fatigue is often performed using electromyography⁷⁾; however, this measurement requires specialized machinery and skilled procedures, making immediate assessment impractical. In contrast, there have been many reports on the measurement of muscle fatigue using the blood lactate acid (BLA) concentration as an index in athletes^{8, 9)}. There are several methods for measuring the BLA concentration^{10, 11)}, among which the use of commercially available simple lactate analyzers is inexpensive and simple. A simple lactate analyzer requires a small amount of blood from the fingertip to be absorbed by a dedicated sensor, and the results are available within 15 s. A rapid understanding of muscle fatigue using a simple BLA-measuring instrument is useful for promoting early recovery from muscle fatigue.

Many studies in the field of sports have measured BLA after unilateral upper limb exercise, and the measurement site has often been the left or right fingertip^12,13,14); however, to date, no report has considered the validity of the measurement point. We believe that it may be important to clarify the BLA measurement point. When measured on the motor limb, abnormally high levels of BLA may occur owing to the incorporation of sweat, or it may be difficult for participants to perceive muscle fatigue or pain owing to paresthesia of the involved limb.

This study aimed to examine the validity of using the left and right fingertips to measure BLA concentrations following unilateral upper limb exercises.

PARTICIPANTS AND METHODS

Forty healthy young male participants were recruited for this study. We excluded those with a current or past history of orthopedic diseases, paralysis, or neurological disease accompanied by pain in the upper limb or alcohol allergy, as well as those taking vitamin C supplements daily. Table 1 shows the physical characteristics of the participants included in the study. This study was approved by the Medical Ethics Committee of Shinshu University (approval no. 4289). Written consent was obtained from all participants.

Table 1. Participants’ physical characteristics (n=40).

Age (years)	Body height (cm)	Body weight (kg)
20.9 ± 2.5	172.0 ± 5.8	63.8 ± 7.6

Values are expressed as the mean ± standard deviation.

The BLA and maximum voluntary contraction (MVC) were measured in 40 participants who did not meet the exclusion criteria. We examined the BLA level before, immediately after the fatigue task, and at 90, 180, and 270 s later. Furthermore, we measured the MVC before the fatigue task and after all measurements were completed and used it to objectively evaluate fatigue.

The puncture site was sterilized with alcohol and dried thoroughly. A small amount of blood was collected by self-puncturing the left and right fingertips with as little time difference as possible (Naturalet Petit, ARKRAY Co. Ltd., Inc., Kyoto, Japan). Blood was drawn into a dedicated sensor and measured (ARKRAY Co., Ltd., Inc.). The results were recorded for 15 s.

We measured the maximum muscle strength of the wrist joint flexion with the non-dominant hand using a handheld dynamometer (#Tas F-1; ANIMA Co. Inc., Tokyo, Japan). Participants with 20–25% maximum muscle strength were provided with 5 kg dumbbells, those with 25–30% were provided with 7 kg dumbbells, and those with >40% were provided with 9 kg dumbbells. Each wrist dorsiflexion was performed for 2 min, once per second.

For data analysis, we used BLA levels at baseline, immediately after exercise, and at 90, 180, and 270 s after exercise, and the MVC before and after exercise. We analyzed time-dependent comparisons of the BLA and MVC on the same side of the upper limb as in-condition comparisons, as well as comparisons of the left and right BLA at the same time between conditions.

In-condition comparisons were performed using the Wilcoxon test for two-condition comparisons and the Kruskal–Wallis test for comparisons between conditions. The post-hoc analysis was performed using Friedman’s test. The significance level was set at p<0.05. All the statistical analyses were performed using SPSS version 25 (IBM Corp., Armonk, NY, USA).

RESULTS

In the in-condition comparison, there was a significant difference between the BLA with pre-measurement values and the measured values immediately after exercise and at 90, 180, and 270 s after exercise for both the motor (Fig. 1A) and non-motor (Fig. 1B) limbs (immediately, p<0.001; 90 s, p<0.001; 180 s, p<0.001; 270 s, p<0.001).

Fig. 1.

In-condition comparison of the blood lactate acid (BLA) of each limb (n=40).

A. In-condition comparisons of the motor limb. The post-exercise BLA measurement values were significantly higher than that of the pre-exercise value. B. In-condition comparison of the non-motor limb. The non-motor limb BLA was significantly higher in each of the post-exercise measurements than in the pre-exercise measurement.

In the wrist flexor condition, the MVC showed a significant difference before and after the completion of all measurements (p<0.001) (Fig. 2).

Fig. 2.

Comparison of the maximum voluntary contraction (MVC) before and after exercise (n=40).

The MVC after exercise was significantly lower after exercise than before exercise. The effectiveness of the load on the muscle fatigue task was confirmed.

Figure 3 shows the measurement results for the left and right BLA levels. In the comparison between conditions, no significant difference was observed in the BLA levels immediately or at 90, 180, or 270 s after exercise (immediately, p=0.389; 90 s, p=0.553; 180 s, p=0.343; 270 s, p=0.985).

Fig. 3.

Between-condition comparison of the blood lactate acid (BLA) of each limb (n=40).

A comparison of the BLA conditions. No significant differences were observed between conditions. BLA peaks were observed immediately after exercise in the motor limb and 90 s after in the non-motor limb.

BLA levels over time differed by approximately 90 s between the peaks of BLA levels in the motor and non-motor limbs. The ratio of the peaks of the motor and non-motor limbs was 1:0.85.

DISCUSSION

In the in-condition comparison, there were significant increases in the BLA between the pre-measurement values and the values measured immediately and at 90, 180, and 270 s later in the motor and non-motor limbs. There were also significant decreases in the MVC before and after the completion of all measurements. These results suggest that the exercise task load was appropriate, and sufficient muscle fatigue occurred. Therefore, we concluded that the increase in the BLA was due to muscle fatigue rather than measurement errors.

At all measurement points, there was no significant difference between BLA values measured on the motor limb and those on the non-motor limb. Based on these results, it is possible that the BLA measured the recovery of muscle fatigue from unilateral upper limb exercise, even if blood was collected from either hand at 270 s after exercise. This suggests that it is not necessary to collect blood from the injured upper limb when assessing muscle fatigue caused by muscular training after a sports injury in one upper limb. Several studies have suggested that blood lactate does not affect sweat lactate^15,16,17). On the other hand, Fellmann et al.¹⁸⁾ and Pilardeau et al.¹⁹⁾ concluded that blood lactate does affect sweat lactate. Furthermore, a recent report showed that a significant increase in the lactate concentration in sweat from the exercised muscle region was observed simultaneously with an increase in the blood lactate concentration during exhaustive exercise²⁰⁾. Until recently, there have been several views on the relationship between sweat lactate and blood lactate, and research has been conducted on various exercise tasks. By contrast, in our study, blood lactate levels were measured using a simple device because immediate measurements were required. Therefore, BLA could not be distinguished from sweat lactic acid owing to equipment limitations. However, in this study, avoiding as much sweat as possible resulted in this being an accurate measurement method. Under these conditions, the results of our study indicate that measurement was possible even with blood collection from non-motor limbs, suggesting that a simple method to suppress the effects of sweat can be proposed.

From the peak values of the BLA measurements, we found a different trend at approximately 90 s between the motor and non-motor limbs. If we wanted to know exactly when non-motor limb muscle fatigue peaked based on the BLA, we had to consider that the measurement time was 90 s after exercise, not immediately after exercise. Furthermore, in this study, the peak non-motor limb value could be expressed as 0.85 times the peak motor limb value. Although this was only a prediction, it may be possible to consider how much the BLA should have been shown in the motor limb when measuring the peak muscle fatigue value in the non-motor limb.

In recent studies, the BLA has often been examined in the earlobes. BLA from the earlobe is easy to use because it does not sweat and is less painful; however, this value was lower than that of the fingertip BLA. We also measured the value from the ear lobe complementarily in our exercise protocol; however, the value was very low, and the peak was not revealed (data not shown). High-intensity exercises are suitable for an earlobe measurement study²¹⁾.

This study has two limitations. First, the puncture times for the left and right fingers were not the same because self-puncture was used as the principle of the BLA measurement. It was impossible for others to puncture due to ethical regulations; therefore, we did not perform simultaneous left and right punctures. Secondly, we did not consider the involvement of the BLA in central fatigue. Recent studies have shown that the BLA acts on the central nervous system, causing pain²²⁾ and pressor responses²³⁾. In this study, we did not consider the effects of central fatigue because the exercise was of relatively low intensity. However, since the post-exercise MVC decreased significantly, it may be necessary to consider central fatigue even if the exercise is not of high intensity.

In conclusion, we found no significant difference between the left and right BLA fingertips. When a healthy male participant underwent an exercise task at 60 bpm for 2 min at a load determined based on the MVC, both measurement sites were considered appropriate. However, there was a time difference between the BLA peaks; therefore, the measurement time should be considered when measuring the muscle fatigue peak using the BLA.

Funding

This research was supported by Grants-in-Aid for Scientific Research (19K12862 to H.N. and 22K17537 to S.O.).

Conflict of interest

The authors have no conflicts of interest.